1. Field of the Invention
The present invention relates to an oil-free screws compressor apparatus, and particularly an oil-free screw compressor apparatus having an improvement of separating oil mist included in the exhaust gas in a gear case of the apparatus.
2. Description of the Prior Art
Generally, a screw compressor includes a male screw rotor and a female screw rotor engaging with each other to rotate in a rotor casing so that the gas, typically, air suctioned from a suction side thereof into the rotor casing is compressed and then discharged therefrom.
In an usual screw compressor, oil is used to seal, lubricate and cool between both rotors and between each rotor and the rotor casing, respectively.
On the other hand, in an oil-free screw compressor, for the purpose of obtaining compressed air including no percentage of oil, oil is not introduced into a rotor casing at all, and as a male and a female rotors hold a narrow gap between both rotors in non-contact state, timing gears mounted on shafts of both rotors and located at the outside of the rotor casing, are engaged with each other in such a manner as to make both rotors rotate in non-contact, synchronous and high speed state. Both rotor shafts are supported by bearings located outside of the rotor casing, with these bearings and timing gears being lubricated by oil. A visco-seal of non-contact type prevents oil from entering into the rotor casing and a carbon-seal of non-contact type suppresses the leakage of air from inside of the rotor casing, with the seals being mounted on the rotor shafts at the suction side and the discharge side of the rotor casing. Further, the rotor shafts having a cooling hole axially passing therethrough so that oil is introduced into the hole through an oil suppling nozzle for cooling located at one end of the rotor shaft and the oil flows out of other end of the rotor shaft to cool the rotor, with cooling water flowing at the periphery of the rotor casing. The inner structure of the above-mentioned screw compressor is well-known.
In the compressor apparatus of FIG. 2, an oil-free screw compressor 1, including the inner structure described above is mounted on a gear case 2, in which a rotor shaft of the compressor 1 is over-driven by an over-drive gear 3 through a gear shaft driven by a pulley 11 to be rotated in the predetermined high-speed rotation. A lower portion of the gear case 2 serves as an oil-reservoir 9 from which the oil suctioned to a oil pump 4 through an oil filter 5, is cooled in an oil cooler 7 to be supplied to a rotor bearing inside of the compressor 1, the oil suppling nozzle for cooling the inner portion of the rotors, timing gears and the overdrive gear 3 etc. to lubricate the same.
Subsequent to lubricating the rotor bearing at the exhaust side, timing gear etc., the oil inside of the compressor 1 is discharged from the oil discharging port 8 to be recovered in the gear case 2 through the oil discharging pipe 12. And, subsequent to lubricating the rotor bearings at the suction side of the compressor 1 the oil is discharged from the oil discharging port 10 to be recovered in the gear case 2. Further, the oil introduced into the cooling hole in the rotor shaft from the oil supplying nozzle for cooling is recovered in the gear case 2 through the end of the rotor shaft at the suction side thereof. Therefore, oil smoke is generated in the gear case 2. On the other hand, since the visco-seal located in the compressor 1 is of a no-contact type and it is necessary to suppress back pressure (i.e. the inner pressure of the gear case 2) at low value thereof in order to maintain the performance of the compressor 1, the inner pressure of the gear case 2 is suppressed at low value thereof by conducting the air inside of the gear case 2 into the exhaust pipe B connected to the gear case 2. Since a very small amount of air leaking from the rotor casing in the compressor 1 flows into the gear case 2 through the exhaust pipe 12, the oil smoke in the gear case 2 flows into the gear case exhaust pipe B. An outlet A of the gear case exhaust pipe B projects to the outdoors so that the oil mist 13 does not enter into an air suction port of the oil-free screw compressor 1.
The above-mentioned prior art are disclosed in Japanese Patent Laid-Open No. 59-51190 and No. 59-51189 with respect to the seal structure of a compressor, in Japanese Patent Laid-Open No. 59-79093 with respect to the casing structure of a compressor, and in Japanese Patent Laid-Open No. 59-93974 with respect to a driving system of a compressor.
It is undesirable to exhaust the oil mist from the outlet of the gear case exhaust pipe B, even if the oil mist is exhausted to the outdoors. Further, it may impossible to exhaust the oil mist to the outdoors in location such as, for example a basement. Therefore, a filter element for removing the oil mist is mounted on the gear case exhausting pipe B, and a suction fan is mounted on the second or downstream side of the filter element.
However, in the above-mentioned prior art with respect to the removal of the oil mist in the gear case exhaust pipe, the pressure loss is increased as a part of the oil is collected in the filter element. Accordingly, there is a problem that the inner pressure of the gear case 2 exceeds the pressure of 40-100 mmH.sub.2 O which is in limit of the performance in the visco-seal inside of the compressor 1. Although it may be possible to suppress the pressure loss to a certain extent by increasing filtering area of the filter element and to suppress the increasing speed of the pressure loss, the sizes of a fan and a filter element are remarkably increased when compared with the compressor. Thus, this is not a practical approach. Also, although it may be possible to remove the oil mist by attaching an electric dust collector etc. to the compressor apparatus, this approach makes the overall compressor system large and expensive, and also is not a practical approach.